Heating element for high-voltage cathodes



. June 8, 1937. G. LIEBMANN 5 HEATING ELEMENT FOR HIGH VOLTAGE CATHODES Filed Sept. 29, 1936 Patented June 8, 1937 sm'res orrics HEATING ELEMENT FOR HIGH-VOLTAGE CATHODES Gerhard Liebmann, Berlin, Germany 3 Claims.

As well known, high-voltage cathodes are understood to be cathodes which may be heated by the mains in direct fashion without the interposition of throttling resistances, or at least those 5 in which by the series connection of a few cathodes of this nature it is accomplished that the full mains voltage may be applied thereto. Since present-day high-emission cathodes require merely a small heating output, the heating element of the cathode may be furnished with a high resistance for the purpose of obtaining very small intensity of the heating current. In the cathodes of this kind hitherto known it has been necessary to pass a very thin and highly heat-resisting wire (for example tungsten) backwards and forwards in numerous windings through the small tube or variously to coil the wire. In this way, however, thickness of the wire and constructions of the heating element are arrived at which are extremely difficult to deal with in manufacture.

A novel and fundamentally different method according to the invention consists in the fact that in place of the very thin and long metallic wire there is employed a straight semi-conductor rod.

Successful attempts have already been made heretofore to employ carbon filaments for heating indirect cathodes. In these, however, the re- 3 sistance cannot be selected sufliciently high in order, with the cathode dimensions usual at the present time, to enable high-voltage cathodes to be produced.

On the basis of the present invention, however,

it is possible to produce high-voltage cathodes having semi-conductor heating elements. The heating element according to the invention consists of a very thin ceramic rod (for example, aluminum oxide), which is coated with a thin layer of so-called hard carbon. This hard-carbon layer is produced by the decomposition of benzole vapour at high temperature and may be regulated practically withindesired limits by selection of the temperature, the gas pressure and the period of reaction. It is quite readily possible in accordance with this process to produce cathodes which, with the dimensions usual at the present time, may be heated in direct fashion by lighting mains of 220 V. The method of producing this layer of carbon is known per se.

The deposit may be obtained, for example, by introducing the ceramic rods to be treated into a refractory furnace pipe, which has been evacuated and filled with benzole vapor, whereupon the entire receptacle is heated over a lengthy evenly through a short heating zone.

period of time to a decomposing temperature of approximately 1,000 C. At the same time, in the manner known per se, the rods to be treated may also be allowed to rotate. There may also be employed the known methods for the continuous application of carbon. In place of the decomposition of benzole it is also possible to employ for this purpose any other suitable hydrocarbon, in the same manner as it is also possible to allow the current of hydrocarbon to flow through the reaction vessel, whereby it may also be diluted with a neutral gas. It is also possible in place of a separately introduced heating element, to employ the small insulating tube contained in the majority of cathodes as support for the layer of carbon, the deposit of car bon being produced on its interior. In this connection, for example, the procedure may be such that an insulating tube of, say, 1 metre in length is' traversed by benzole vapor with preclusion of the air whilst the entire tube is being moved At the hot points there occurs a deposition of carbon within the tube. The long tube coated with carbon on the inside is later cut to the desired length.

For the purpose of making contact the carboncoated heating elements are furnished preferably with clamped on shoes, whereby the contact resistance may be decreased by an intermediate layer of graphite or by a metallic coating. Cathodes having the described heating elements are distinguished by a particularly even temperature. If it is desired to keep the ends of the cathode cool, this may be readily accomplished by extending the metallic coating at the ends. The heating rod commences to glow only proceeding from the end of the metallic coating. By variable application of carbon to single parts of the heating rod, which may take place, for example, by corresponding distribution of the temperature in the furnace, it is possible to obtain a certain desired disposal of the temperature over the surface of the cathode.

In the drawing there is shown diagrammatically the assembly of the cathode having the heating element in accordance with the inven tion; Fig. 1 is a sectional view along the axis and Fig. 2 a sectional view cut vertically to the axis, while Fig. 3 shows a modification in a sec tion vertically to the axis.

In the drawing l is the semi-conductor rod, 2 the metallic coating at the ends, 3 the connecting caps, 4 a small insulating tube, 5 the equipotential layer, 6 the emissive layer and l the hard carbon layer. Figs. 2 and 3 show the ceramic support I having the layer of hard carbon 1, whereby in Fig. 2 the hard-carbon layer is applied to the outside, whilst in Fig. 3 it is provided in the interior of the small tube.

To impart to the heating element an electrically completely stable condition it is desirable to maintain the same for a certain period of time at a high temperature (approximately 1,000 C.) in a vacuum or a neutral atmosphere, either by conduction or by external heating.

I claim:

1. A highly emissive cathode of the indirectly heated type consisting of a tubular semi-conductor rod of ceramic material furnished over its entire length with a hard carbon layer and furnished at the ends of the cylindrical surface with metallic coatings to which metallic connecting caps are fitted, a second insulating tubular rod fitted over said first rod, said second rod being furnished with an equipotential layer and a highly emissive layer.

2. A highly emissive cathode of the indirectly heated type consisting of a tubular semi-conductor rod of ceramic material furnished over its entire length on the outside with a hard carbon layer and furnished at the ends of the cylindrical surface with metallic coatings to which metallic connecting caps are fitted, a second insulating tubular rod fitted over said first rod, said second rod being furnished with an equipotential layer and a highly emissive layer.

3. A highly emissive cathode of the indirectly heated type consisting of a small insulating tubular rod of ceramic material furnished over its entire length in the interior with a hard carbon layer and furnished at the ends of the inner cylindrical surface with metallic coatings, to which metallic coating caps are fitted, said rod being furnished at the outer surface with an equipotential layer and a highly emissive layer.

GERHARD LIEBMANN. 

